This invention relates to a method and apparatus for reducing the amount of nuisance or volatile compounds that are normally concentrated within the pyroprocess equipment of a cement plant incorporating a rotary kiln in which cement raw feed is burnt to product cement clinker as a stage in cement manufacture.
It has become general practice to effect preheating and hence a partial calcination of the raw cement feed before the raw feed is introduced into the rotary kiln for the main heat treatment. This preheating stage serves to reduce overall fuel consumption. In the main heat treatment, the calcination is completed by a burning or sintering process.
Nearly all cement raw materials contain to some extent compounds with a content of alkali, chlorine or sulphur and these compounds give rise to various drawbacks if their quantity is too great. Under the high temperature conditions encountered, these compounds enter into a gaseous state and are often referred to as "volatile" compounds. The volatile compounds are concentrated within the pyroprocess system (principally the cement kiln) through a volatilization and recondensation process and can disrupt production stability and also diminish cement quality. The most common compounds encountered are K.sub.2 O, Na.sub.2 O, Cl, and SO.sub.3.
Various attempts have been made in the prior art to reduce the detrimental effect of these nuisance or volatile compounds and certain of these attempts have to a degree been successful but not without causing other forms of drawbacks. For example, in Canadian Pat. No. 1,021,561 issued Nov. 29, 1977 to Christiansen, there is disclosed a process wherein a flow of hot gases is removed from the kiln and such flow is exposed to a "rain" of freely falling condensation bodies which are cold relative to the temperature of the gas flow so that at least a portion of the volatiles in the gas flow condense on such condensation bodies. The condensation bodies are then subjected to impacts or the like to loosen the coating material thereon following when the condensation bodies (which may be in the form of iron balls) are recirculated. The process, among other things, obviously involves the use of relatively complex and costly equipment and adds significantly to the total number of process steps involved.
Many existing installations attempt to reduce the volatile recirculation within the kiln and the volatiles concentration in the cement product through by-passing (eliminating from the pyroprocess system) a percentage of the hot kiln gases at the feed end of the kiln. Since the dust contained in the by-pass gases is enriched in volatile condensate, the potential efficiency of a by-pass system for volatile removal is relatively high. Accordingly, various forms of by-pass systems are in use in many modern cement plants. Unfortunately, due to their design and placement relative to the pyroprocess system, the efficiency of such systems is often severely reduced due to the ingestion by the by-pass system of raw cement feed to the kiln, which raw cement feed obviously does not contain significant concentrations of the volatile material. These existing by-pass systems generally consist simply of a port in the stationary (riser duct) structure at the feed end of the rotating kiln. This port is put under a lower static pressure than that existing in the riser duct such that the desired amount of kiln gases and associated volatile enriched dust is eliminated from the pyroprocess system. The by-pass gases are cooled by mixing with ambient air and are separated from the dust through the use of standard dust separating equipment. However, due to the by-pass port being in close proximity to the system for supplying fresh raw feed to the kiln, such existing by-pass designs are often highly inefficient due to the ingestion of fresh feed as noted above.